Fuel assemblies in certain boiling water nuclear reactors contain fuel rods and water rods. Typically, cladding containers of both types of rods are made of zirconium alloys such as Zircaloy-2 and Zircaloy-4. The fuel rods contain fuel material, such as uranium dioxide pellets whereas the water rods have water flowing through them.
Under the conditions prevailing in an operating nuclear reactor, zirconium alloys exhibit a certain amount of irradiation growth. The amount of irradiation growth depends on many factors, including the alloy material, the temperature, neutron energy, and the neutron flux. Mechanical interaction between fuel pellets and the cladding container introduces a growth component in the fuel rods not present in water rods. This introduces an irradiation growth differential between fuel rods and water rods. As the radiation exposure increases, pellet-cladding mechanical interaction (PCMI) increases and as a result the difference in irradiation growth between fuel rods and water rods increases.
In such a nuclear fuel assembly the fuel rods are positioned lengthwise between upper and lower tie plates in which the ends of the fuel rods are secured, for example by means of end plugs having studs fitted into holes in the tie plates. Water rods are positioned lengthwise between the tie plates and are likewise secured by means such as end plugs with studs fitted into corresponding holes in the tie plate.
There is typically an axial irradiation growth differential among fuel rods and water rods. A small axial growth differential is accommodated, for example, by incorporation of expansion springs between the upper tie plate and the upper end of the fuel rod cladding. The expansion springs are disposed around a portion of the end plug studs.
The distance between the upper and lower tie plates increases according to the axial irradiation growth of the fuel rods. There is concern that the difference in axial irradiation growth between fuel rods and water rods may become too great to be accommodated by the expansion springs and may cause one or more end plug studs of water rods to release from the upper or lower tie plate.